The Earth's Atmosphere

Source: NASA

The majority of the Earth's atmosphere is composed of a mixture of only
a few gases-nitrogen, oxygen, and argon; combined these three gases comprise
more than 99.5% of all the gas molecules in the
atmosphere.
These gases which are most
abundant
within the atmosphere exhibit almost no effect on warming the earth and its
atmosphere since they do not absorb
visible
or
infrared radiation.
However, there are minor gases which comprise only a small portion of
the atmosphere (about 0.43% of all air molecules, most of which are water
vapor at 0.39%) that do absorb infrared radiation. These "trace" gases
contribute substantially to warming of the Earth's surface and atmosphere
due to their abilities to contain the infrared radiation emitted by the
Earth (see below for details on the Greenhouse Effect). Since these trace
gases influence the Earth in a manner somewhat similar to a greenhouse,
they are referred to as
GreenHouse Gases, or GHGs.

Composition of Earth's Dry Atmosphere (as of 2009)

Nitrogen

78.1%

Oxygen

20.9%

Argon

.9%

Carbon Dioxide

.039%

Methane

.00018%

Nitrous Oxide

.000032%

Sulfur Hexafluoride

.00000000067%

Water vapor is the most important GHG, since globally it is the most abundant of these gases,
although it varies from 0-3% in a given location. NOAA's
Carbon Cycle Greenhouse Gases (CCGG) group
is concerned with the abundances of many of the other GHGs, since humans have a dominant
role in the growing atmospheric concentrations of these gases. The gases measured by the CCGG include
carbon dioxide
(the second most important GHG),
methane,
nitrous oxide,
sulfur hexafluoride,
ozone, and a few others. While these gases
constitute only a tiny fraction of Earth's very large atmosphere,
their amounts are sufficient to absorb a major fraction of the
infrared light in the atmosphere.

Influential Greenhouse Gases

Carbon Dioxide (CO2)
is a colorless, odorless gas consisting of molecules made up of
two oxygen atoms and one carbon atom. Carbon dioxide is produced
when an organic carbon compound (such as wood) or fossilized
organic matter,
(such as coal, oil, or natural gas) is burned in the presence of
oxygen. Carbon dioxide is removed from the atmosphere by carbon
dioxide
"sinks",
such as absorption by seawater and photosynthesis by
ocean-dwelling plankton and land plants, including forests and
grasslands. However, seawater is also a
source,
of CO2 to the atmosphere, along with land plants, animals, and soils,
when CO2 is released during respiration.

Methane (CH4)
is a colorless, odorless non-toxic gas consisting of molecules
made up of four hydrogen atoms and one carbon atom. Methane is
combustible, and it is the main constituent of natural gas-a
fossil fuel. Methane is released when organic matter decomposes in
low oxygen environments. Natural sources include wetlands, swamps
and marshes, termites, and oceans. Human sources include the
mining of fossil fuels and transportation of natural gas,
digestive processes in ruminant animals such as cattle, rice
paddies and the buried waste in landfills. Most methane is broken
down in the atmosphere by reacting with small very reactive
molecules called hydroxyl (OH) radicals.

Nitrous oxide (N2O)
is a colorless, non-flammable gas with a sweetish odor, commonly
known as "laughing gas", and sometimes used as an
anesthetic. Nitrous oxide is naturally produced in the oceans and
in rainforests. Man-made sources of nitrous oxide include the use
of fertilizers in agriculture, nylon and nitric acid production,
cars with catalytic converters and the burning of organic matter.
Nitrous oxide is broken down in the atmosphere by chemical
reactions driven by sunlight.

Sulfur hexafluoride (SF6)
is an extremely potent greenhouse gas. SF6
is very persistent, with an atmospheric lifetime of more than a
thousand years. Thus, a relatively small amount of SF6
can have a significant long-term impact on global climate change.
SF6
is human-made, and the primary user of SF6
is the electric power industry. Because of its inertness and
dielectric properties, it is the industry's preferred gas for
electrical insulation, current interruption, and arc quenching (to
prevent fires) in the transmission and distribution of
electricity. SF6
is used extensively in high voltage circuit breakers and
switchgear, and in the magnesium metal casting industry.

The Greenhouse Effect

Source: Barb Deluisi, NOAA

Many of the atmospheric
trace gases, despite their relatively minor abundances, have a
significant influence on Earth's
climate,
due to a phenomenon called the
"Greenhouse Effect".

The Sun ultimately
drives Earth's climate by emitting energy in the form of sunlight.
Sunlight is solar radiation mostly in the form of visible and a
smaller portion as
ultraviolet
(UV) energy. This is also
called shortwave radiation. Clouds and the Earth's surface reflect
some of this incoming solar radiation back out to space
(approximately 30%), some (mostly UV) is absorbed by the atmosphere
(about 20%), and the remaining half is absorbed at the Earth's
surface. Sunlight absorbed by Earth's surface acts to warm the
surface.

Source: Barb Deluisi, NOAA

The solar energy that
has been absorbed by Earth's surface is then emitted in a different
form. Since Earth is much cooler than the Sun, it emits weaker
radiation with longer wavelengths, in the infrared range. Some of
this infrared radiation passes through the atmosphere unimpeded, but
the majority is absorbed by GHGs and then reemitted in all
directions-towards space, to other GHG molecules, and back to
Earth's surface. In this way, GHGs block most of the infrared
radiation within the atmosphere that would otherwise escape directly
into space.

This process is naturally occurring and beneficial, as it
maintains favorable living conditions for Earth's microbial, animal
and plant inhabitants. The global average temperature is 14°C
(57°F), which is approximately 33°C (59°F) warmer than
temperatures would be without an atmosphere and GHGs. Due to their
ability to absorb infrared radiation, GHG molecules have a
significant impact on Earth's climate by acting as a barrier for
escaping "heat".

For over a century,
scientists have realized that concentrations of atmospheric gases may
significantly affect Earth's climate through this process.
Scientists have been measuring GHGs in the atmosphere for more than
50 years. Charles Keeling began continuous measurements of CO2
concentrations in 1958 and others, including NOAA scientists,
followed shortly thereafter. Today, there is unequivocal scientific
evidence that the abundance of these gases is increasing in the
atmosphere. Evidence includes decades of carefully calibrated, global
measurements of these trace gases, combined with measurements of
"old" air preserved in bubbles embedded in ice cores and
measurements of carbon
isotopes,
in tree rings (from which past
atmospheric CO2 can be reconstructed). This increase in
atmospheric GHGs has a significant impact on Earth's climate
because Earth's incoming and outgoing radiation is out of balance
--which forces the climate to change.

Source: Barb Deluisi, NOAA

As the concentrations
of GHGs increase within the atmosphere, more infrared radiation is
absorbed and less escapes directly to space, resulting in amplified
warming. This is called the
Enhanced Greenhouse Effect.

Note: This atmospheric
process is referred to as the Greenhouse Effect, since both the
atmosphere and a greenhouse act in a manner which retains energy as
heat. However, this is an imperfect analogy. A greenhouse works
primarily by preventing warm air (warmed by incoming solar radiation)
close to the ground from rising due to
convection,
whereas the
atmospheric Greenhouse Effect works by preventing infrared radiation
loss to space. Despite this subtle difference, we refer to this
atmospheric process as the Greenhouse Effect and these gases as
Greenhouse Gases because of their role in warming the Earth.

The Carbon Cycle

Of the GHGs, CO2
is of greatest concern because it contributes the most to the
Enhanced Greenhouse Effect and
climate change.
For this reason, scientists (at NOAA and elsewhere) have been studying this
molecule carefully and attempting to quantify its abundance in the
atmosphere and track how and why it changes. The CO2
molecule is involved in a complex series of processes called the
carbon cycle,
where the
carbon
atom within the molecule
moves between many different natural reservoirs. As carbon is
transferred between
reservoirs,
processes which release CO2
into the atmosphere are called sources, and processes which remove
CO2 from the atmosphere are called sinks.

Source: Barb Deluisi, NOAA

Carbon is continuously exchanged and recycled among the reservoirs through natural
processes. These processes occur at various rates ranging from
short-term fluctuations which occur daily and seasonally to very
long-term cycles which occur over hundreds of millions of years. For
example, there is a clear seasonal cycle in atmospheric CO2
as plants
photosynthesize
during the growing season, removing
large amounts of CO2.
Respiration
(from both plants
and animals) and
decomposition
of leaves, roots, and organic
compounds release CO2 back into the atmosphere. On a scale
spanning decades to centuries, CO2 levels fluctuate
gradually between the ocean and atmospheric reservoirs as ocean
mixing occurs (between surface and deep waters) and the surface
waters exchange CO2 with the atmosphere.
Much longer cycles also occur, on the scale of
geologic time,
due to the deposition and weathering of carbonate and silicate rock.
Carbonate rocks like limestone are formed from the shells of marine
organisms buried on the ocean floor, and they are chemically eroded
by reaction with CO2 (remember that CO2 mixed
with water is an acid) in the air and in soils. Silicate rock reacts
with carbonate rock deep underground, producing CO2 gas
coming out of volcanoes.
Fossil fuels
form a relatively small part of these natural geologic cycles.

Carbon Reservoirs and Exchange

At time scales of most
interest to humans (years to decades to centuries) the atmosphere
exchanges carbon with three main reservoirs: the
terrestrial biosphere,
the oceans, and fossil fuels.

Source: NOAA

Terrestrial Biosphere

Source: NOAA

The terrestrial
biosphere defines the part of the earth system that supports
organisms living on land, and includes plants, animals, soil
microbes, and decomposing organic material. Since carbon is a main
component of organic molecules that are the building blocks for all
life, a large amount of organic material is stored in the terrestrial
biosphere-it is one of the main reservoirs for carbon. In addition,
there is a large amount of carbon exchanged seasonally between the
terrestrial biosphere and the atmosphere. Surface exchanges (or
"fluxes"
) result from organisms living within the
terrestrial biosphere, and they naturally include both sources and
sinks. Some of the terrestrial biosphere's major sources of
atmospheric CO2 include respiration by land
biota
(plants, animals, microorganisms, humans, etc) and the burning and
decomposition of organic material. The removal of atmospheric CO2
by the terrestrial biosphere occurs through photosynthesis. Plants
use CO2 from the atmosphere to build food in the form of
organic matter--which in turn becomes food for microbes, fungi,
insects, and higher organisms. Human activities have a considerable
impact on the terrestrial biosphere's ability to remove or emit
carbon dioxide through practices such as
deforestation
and other forms of land management.

Oceans

Source: NOAA

The oceans continuously
exchange CO2 with the atmosphere. Due to the large surface
area of the oceans and the high solubility of carbon dioxide in water
(which creates
carbonic acid
), the oceans store very large
amounts of carbon - about 50 times more than is in the atmosphere
or terrestrial biosphere. Each year, some of that carbon is released
to the atmosphere, and a similar amount is taken back up into the
oceans (although the two processes might occur in different parts of
the world's oceans). In addition, organisms within the
marine biosphere
photosynthesize and respire CO2. Due to the
slow rate of mixing between surface and deep ocean waters, only the
surface waters are responsible for short-term changes of atmospheric
CO2. As the atmospheric CO2 concentration
increases, the ocean sink also increases slightly. The oceans will
eventually absorb the majority of the CO2 released from
human activities, but this will take thousands of years. CO2
in the form of carbonic acid is a weak acid, and there are profound
implications on marine ecosystems due to the increasing acidity of
the oceans.

Fossil Fuels

Source: EPA

Over the course of
millions of years, as biomass from dead plants and microorganisms
accumulated in sediments and was subjected to high temperature and
pressure deep below Earth's surface, organic remains from the
biosphere
(both terrestrial and marine) have been converted to
fossil fuels (coal, oil, and natural gas). However, since the
beginning of the
Industrial Revolution
in the 1800s, humans
have been burning these fossil fuels, releasing the carbon from them
back into the atmosphere as CO2. Processes that took
millions of years to remove carbon from the biosphere have
been reversed so that the same carbon is being released at
unprecedented rates as a result of human activities. Atmospheric CO2
levels have increased 38% [as of 2009] since
Preindustrial
times and are higher than at any time in the past 800,000 years.

Currently, atmospheric
CO2 levels continue to rise at an accelerating rate as
humans burn fossil fuels at increasing rates. In human terms, the CO2
emitted by the combustion of fossil fuels (along with cement
manufacturing and other human activities) remains "forever" due
to the stability and longevity of CO2 within the
atmosphere and oceans. This will have significant implications on the
Earth System, as the resulting radiation imbalance from the Enhanced
Greenhouse Effect will noticeably alter the global climate for
centuries to millennia.